High Energy Physics - Phenomenology

Title:
(A)symmetries of weak decays at and near the kinematic endpoint

Abstract: At the kinematic endpoint of zero recoil physical momenta are parallel which
leads to symmetries in the decay distributions. We implement this observation
for decays of the type $A \to (B_1 B_2) C$ by extending the helicity formalism
to include an unphysical timelike polarisation. The symmetries of the helicity
amplitudes are worked out for a generic dimension six Hamiltonian for a $B \to
V \ell \ell$ decay type. We obtain \emph{exact} predictions for angular
observables, e.g.,for the fraction of longitudinally polarized vector mesons,
$F_L = 1/3$, which may be used to guide experimental analyses. We investigate
the vicinity of the endpoint through an expansion in the three momentum of the
vector meson. New physics can be searched for in the slope of the observables
near the endpoint. Current experimental data on $B \to K^* \ell \ell$ decays
are found to be in agreement with our predictions within uncertainties.
Application to other semileptonic $B$ and $D$ decays, including $B \to V \ell^+
\ell^-$, $V=K^*,\phi, \rho$ and $B \to V \ell \nu$, $V=\rho,D^*$ is
straightforward. For hadronic modes of the types $B \to V p \bar p, V \Lambda
\bar \Lambda, ..$ and $B \to V \pi \pi, V \pi K, .. $ endpoint relations apply
as long as they are not overwhelmed by sizeable final state interactions
between the $V$ and the hadron pair.